Constant vs Variable Torque

[philly]

Can someone please explain the difference between constant and variable torque.

I have read that conveyors are considered constant torque loads. Does this mean that as you increase the speed of the conveyor the torqure requirement stays the same?

I have also read that fans are considered variable torque loads. Does this mean that as you increase the speed the torque requirement increases?

 

[JJWalecka]

AC Motors - Variable torque: AC motors have a speed torque characteristic that varies as the square of the speed. For example, an 1,800/900-rpm electrical motor that develops 10 hp at 1,800 rpm produces 2.5 hp at 900 rpm. Since ac motors face loads, such as centrifugal pumps, fans, and blowers, have a torque requirement that varies as the square or cube of the speed, this ac motor characteristic is usually adequate.

AC Motors - Constant torque: These ac motors can develop the same torque at each speed, thus power output varies directly with speed. For example, an ac motor rated at 10 hp at 1,800 rpm produces 5 hp at 900 rpm. These ac motors are used in applications with constant torque requirements such as mixers, conveyors, and compressors.

JJ

 

[jim dungar]

I have also read that fans are considered variable torque loads. Does this mean that as you increase the speed the torque requirement increases?

Because variable torque loads vary with the square or cube of the speed (as JJ said), attempting to drive these loads faster than base speed can significantly impact the amount of power required.

 

[rcwilson]

Horsepower = (Torque in foot-pounds x speed in rpm)/5250

On a conveyor, it takes the same torque to move the load whether the belt is moving fast or slow. The weight that the conveyor is lifting doesn't change. Torque is constant. Double the conveyor speed and you double the HP needed because torque is constant.

Torque needed to spin a centrifugal fan or pump varies with the speed. It takes more torque to spin the pump fast. At faster speeds the fan builds more pressure. Think of the pressure pushing back against the fan blades increasing the torque needed. Since HP= torque x speed, the HP increases rapidly with speed increase. (Flow varies directly with speed. Pressure varies with the square of the speed. Power needed = pressure x flow. So fan HP varies with the cube of the speed.)

In general:

pumps and fans = variable torque loads (Positive displacement pumps not included.)


conveyors, elevators, machines = constant torque load.

 

[philly]

Horsepower = (Torque in foot-pounds x speed in rpm)/5250

On a conveyor, it takes the same torque to move the load whether the belt is moving fast or slow. The weight that the conveyor is lifting doesn't change. Torque is constant. Double the conveyor speed and you double the HP needed because torque is constant.

I'm assuming that with a conveyor if you add more material or weight then the torque will indeed increase. However if weight is kept the same and belt is increased in speed than torque still stays the same

Does current increase as a function of speeding up a conveyor. If the horsepower increases then I"m guessing the current would too.

Torque needed to spin a centrifugal fan or pump varies with the speed. It takes more torque to spin the pump fast. At faster speeds the fan builds more pressure. Think of the pressure pushing back against the fan blades increasing the torque needed. Since HP= torque x speed, the HP increases rapidly with speed increase. (Flow varies directly with speed. Pressure varies with the square of the speed. Power needed = pressure x flow. So fan HP varies with the cube of the speed.)

What if in addition to the fan being operated faster a damper opens allowing more air to flow. Would the fan then increase its torque per the cube of the speed plus due to the additional air loading?

 

[Besoeker]

Can someone please explain the difference between constant and variable torque.

I have read that conveyors are considered constant torque loads. Does this mean that as you increase the speed of the conveyor the torqure requirement stays the same?

I have also read that fans are considered variable torque loads. Does this mean that as you increase the speed the torque requirement increases?

I assume that this refers to variable frequency inverters?
The constant torque/variable torque classification is one that VFD manufacturers often use.
The difference is usually the overload rating of the VFD
Typically, the constant torque rating is based on a 50% overload capability and the variable torque rating on a 10% overload capability.
Based on manufacturer's data, the same VFD can do 37kW VT and 30kW CT.

 

[Besoeker]

Horsepower = (Torque in foot-pounds x speed in rpm)/5250

On a conveyor, it takes the same torque to move the load whether the belt is moving fast or slow. The weight that the conveyor is lifting doesn't change. Torque is constant. Double the conveyor speed and you double the HP needed because torque is constant.

Torque needed to spin a centrifugal fan or pump varies with the speed. It takes more torque to spin the pump fast. At faster speeds the fan builds more pressure. Think of the pressure pushing back against the fan blades increasing the torque needed. Since HP= torque x speed, the HP increases rapidly with speed increase. (Flow varies directly with speed. Pressure varies with the square of the speed. Power needed = pressure x flow. So fan HP varies with the cube of the speed.)
In general:
pumps and fans = variable torque loads (Positive displacement pumps not included.)
conveyors, elevators, machines = constant torque load.

I agree with all of that.
Your initial formula again reminded me on how much simpler SI (metric) is.

 

[weressl]

Can someone please explain the difference between constant and variable torque.

I have read that conveyors are considered constant torque loads. Does this mean that as you increase the speed of the conveyor the torqure requirement stays the same?

I have also read that fans are considered variable torque loads. Does this mean that as you increase the speed the torque requirement increases?

Centrifugal loads, such as pumps, blowers, fans and agitators are subject to the following laws. The laws explain the power, capacity and speed relationship to each other. These are called variable torque loads.

easy reference: http://www.gouldspumps.com/cpf_0010.html

Constant torque loads are where the load does not change with the speed. The easiest example to understand is the conveyor belt example. It's length is fixed, so is the material on it, therefore the mass to be moved is unchanging weather it is moving at 1fps or 10fps.

Torque is defined as the force needed to move a certain mass.

Power will define how fast that certain mass can be moved.



1HP=550 pound-feet per second. In other words; 1 HP is needed to move 550# one foot in one second.

 

[philly]

Constant torque loads are where the load does not change with the speed. The easiest example to understand is the conveyor belt example. It's length is fixed, so is the material on it, therefore the mass to be moved is unchanging weather it is moving at 1fps or 10fps.

Wouldn't constant torque loads still see an increase in current as a result of running faster since the HP would increase?

Is current more a direct relationship of required torque or horsepower? It looks like current will increase in both CT and VT applications when speed is increase except it will rise at a much faster rate in an VT application and not linearly.

Why do VFD's have a CT and VT rating if current is current. Both a VT load and a CT load can draw excess current for any given amount of time.

 

[Jraef]

Wouldn't constant torque loads still see an increase in current as a result of running faster since the HP would increase?

Yes, it would, because current is more directly related to HP, which is defined by torque AND speed. But the torque required to move that load would remain the same because torque is more directly related to load.

Is current more a direct relationship of required torque or horsepower?

HP, see above.

It looks like current will increase in both CT and VT applications when speed is increase except it will rise at a much faster rate in an VT application and not linearly.

Essentially, yes, because in a VT load, the LOAD is increasing along with speed, so the effect on HP and thus current is exponentially increased. Another term for a Variable Torque load in Europe is a "Quadratic Load", because the equation for power use is quadratic.

Why do VFD's have a CT and VT rating if current is current. Both a VT load and a CT load can draw excess current for any given amount of time.

Because the "excess" current is only seen when you over speed the motor. But in a VT load, as you turn the speed DOWN, the load decreases at the same quadratic rate, meaning that at 1/2 speed, you need only 1/2 x 1/2 x 1/2 the HP, or 1/8th. Therefore, a VT drive is, by definition, never going to need to deliver high power, and therefore high current, when running at lower speeds.

In VFD design, the total circuit efficiency drops with speed because in order to get lower speed, the PWM pattern has the transistors on for longer periods of time in each cycle (because the cycles are slower). That is why when you see a VFD mfr quote that their drive is "97% efficient" they will always qualify that statement by saying that is a full speed. At 1/2 speed, the best are only 95% efficient, most are less. So that means that the heat rejection by the transistors (switching losses) goes up as speed goes down. In a CT drive then, you must size the transistors for the worst case scenario, which is below 1/2 speed with full load. That then means you either have to have a massive heat sink and cooling system, or you have to oversize the transistors compared to their loading at full speed. Most manufacturers are doing a combination of both factors.

In a VT load however, the drive would never see that scenario! So the VFD mfrs can "get away with" using smaller transistors for larger loads, or in actuality, rating a VFD for a higher HP if used on a VT load. The VFD in either case is exactly the same, it is just the max. HP rating that is changing because of the expected load profile.

 

[hillbilly]

1HP=550 pound-feet per second. In other words; 1 HP is needed to move 550# one foot in one second.

1 HP is the force required to LIFT 550 lbs. 1 foot in one second.

I'm sure that's what you meant....Just to clarify for those who might not know that.:smile:

steve

 

[weressl]

1 HP is the force required to LIFT 550 lbs. 1 foot in one second.

I'm sure that's what you meant....Just to clarify for those who might not know that.:smile:

steve

In vacuum, at sea level, to be absolutely accurate. Otherwise dragging it on a well lubricated surface would also qualify to moving it.

 

[gar]

081112-1142 EST

Torque and force are analogous. Force is a linear motion vector and torque is a rotational force vector. If torque or force is applied to something and there is no motion, then there is no power transferred. Force and torque will be related in any system based upon the mechanical system.

A weight on a rope suspended from a pulley is an approximately constant force system when moving at a constant velocity over a limited height change. If the rope is connected to a drum, then at constant velocity the torque to rotate the drum is a constant.

If you have a constant torque load, then you do not want to drive this with a constant torque source. If the driving torque is less than the load, then you have zero velocity. If the driving torque is greater than the load, then velocity approaches infinity. In reality something will limit maximum velocity.

If you have a variable torque load you may want to drive this from a constant torque source.

Your may want a source that has a combined characteristic. Constant speed and variable torque up to a maximum torque, then maybe constant torque and obviously variable speed with a maximum speed limit, or a foldback torque curve.

A permanent magnet DC motor is an approximately constant torque device if the input current is held constant, and an approximately constant speed device if input voltage is held constant.

AC motors are more complicated. But, if under moderate load, then the input current will be approximately proportional to load torque. A better estimate method to use would be to measure input power, output RPM, and calculate torque. If the speed is moderately constant, then torque is approximately proportional to HP.

With respect to internal combustion engines you want to look at both torque and HP curves relative to speed. Advertized power will be at the peak power point, maybe 4000 to 6000 RPM. Engine RPM in overdrive at 70 MPH might be 1900 RPM. An engine with lower peak HP and at a lower RPM compared to another may have a higher torque at 1900 RPM and thus more power at the normal operating conditions at 1900 RPM. Maximum advertized HP may not be your best choice. Diesel engines have good low speed torque characteristics.

.

 

[Besoeker]

Yes, it would, because current is more directly related to HP, which is defined by torque AND speed. But the torque required to move that load would remain the same because torque is more directly related to load.

With respect, I don't think you are correct.
I don't know how easy it is to explain in text, but I'll give it a go.

If the load is truly constant torque (few really are) the stator current will be the same, regardless of speed.
Torque is proportional to the product of stator flux and rotor current
If you operate the inverter with a constant V/f ratio, the magnitude of the stator flux (and current) remain constant. And the magnetising component of the current remains constant.
Over the normal operating range, rotor current is proportional to slip - the difference betwen synchronous speed and actual speed. The same slip at any speed will result in the same rotor current.

 

[Ranch]

total circuit efficiency drops with speed because in order to get lower speed, the PWM pattern has the transistors on for longer periods of time

Huh? more time makes more voltage .....

At any rate, put constant torque in your spec, expect a larger ASD

One of my favourites:

Conveyor in a valley - drops 1000' and then climbs 1100'. When first loaded our ASD needs to hold it back so we don’t go running away when the rock is on the downhill section only, maybe even for a minute. Maybe even at 150% of the ASD rating …

Then the rock feed stops, and we have to take it up the hill at some point with rock only on the uphill section. Maybe even for a minute. Maybe even at 150% of the ASD rating …

So we best design the controls to handle these two conditions, regenerative and motoring, constant torque - 150% OL capacity for 60 seconds

Now let’s say the other 23.9 hours of the day, we have the rock headed downhill pushing the uphill rock. Wow, this thing is like idling, awesome since we had to size CT. Yup, this ASD should last forever if we PM connections and airflow.

VT? Boring, boring ……. Any search engine should return results on “Laws of Affinity”

VT market, centrifugal loads, sell the smallest thing you can get away with because the vast majority of VT money comes from tax payers. Well now how is that for a can of worms …..

 

[philly]

I'm trying to think of loads besides fans and pumps that would be variable toruqe. It sounds like variable torque is anything that has a load increase with speed increase.

What about a grinding mill or a shaker. Although these loads are centrifigual I wouldn't think that they have an increase in load with an increase in speed.

 

[hillbilly]

This may help.

Think in terms of Constant Horsepower and Constant Torque.

1 Horsepower is the amount of force required to lift 550 lbs 1 foot in one second.....or 33000 lbs. 1 foot in one minute.

These values were determined by actual tests using a horse (or a number of average horses). http://www.web-cars.com/math/horsepower.html

Any combination of the lifting speed and weight are accurate as long as the same amount of work is accomplished. ....such as...

It takes the same amount of work to lift 1 pound 1 foot in 1 second as it does to lift 2 pounds 1 foot in 2 seconds, or 2 pounds 1/2 ft. in 1 second....etc.....get the idea?

It's all in the amount of work done verses the amount of time required to do the work.

Constant Horsepower is the amount of work required to do this amount of work continuously....such as a electric motor pulling the same load continuously....it's delivering constant Horsepower.

Torque is a measurement of turning or twisting force around a axis.
1 lb/ft. of torque is the amount of twisting force applied around a axis that has a lever of 1 ft and a applied pressure of 1 lb....my wording.

Say that you have a wrench that is 1 foot long attached to a shaft, and you apply 1 pound of pressure to the end of the wrench.
You are applying 1 lb./ft. of torque to the shaft.

If you keep applying the 1 pound force constantly....You are applying 1 lb./ft. of Constant Torque to the shaft.
It doesn't matter if any work is performed...it is merely a measure of force..either instantaneous, intermittiant or constant (as in the example).

Motors rotate (no pun intended)....so the force that they generate in order to do work is torque.

In the HP example above, since the Motor is delivering a constant amount of work (constant HP)...it is also at the same time delivering constant torque to it's output shaft.

If for example I half the load to .5 lbs. and double the lift speed to 2 feet per second...the required torque on the shaft would remain the same.

If I double the load to 2 lbs. and the lift speed remains 1 foot per second, the Horsepower required would double and the torque required would also double.

These are my own opinions, and were arrived at by actually working with motors and loads...so the terminology may not be entirely accurate.
I didn't learn this from a book.....well maybe some of it.:smile:

Feel free to analyze...and correct me if I'm wrong.
I'm here to learn.

steve

 

[gar]

081113-1318 EST

Steve:

You used the correct units for torque (#-ft), very good, but very few people do because of the incorrect labeling on torque wrenches.

Work is ft-# and torque is #-ft.

.

 

[gar]

081113-1324 EST

philly:

All sorts of loads exist that have variable load torque at constant speed or variable speed.

A vehicle accelerating and then decelerating .

A lathe at constant spindle speed as the the cutting tool moves in or out, or encounters different depths of cut or material hardness. Same for a milling cutter.

A very long rope and pulley as you raise a weight above the earth. Two factors --- less rope on the load side of the pulley and the weight of the mass gets less.

A DC generator with a variable resistance load.

A DC generator with a fixed resistance load will required increased torque to drive it as speed is increased.

A DC generator with a constant power load will require less torque as speed is increased.

.

 

[philly]

081113-1324 EST


A vehicle accelerating and then decelerating .


.

Wouldn't this be the same as a conveyor speeding up and slowing down? The conveyor is constant torque.